Toilet blocks tax free (6)
I believe the answer is:
vacant
'free' is the definition.
(thesaurus)
'toilet blocks tax' is the wordplay.
'toilet' becomes 'can' (synonyms).
'blocks' is an insertion indicator (upper walls off glass inside formal).
'tax' becomes 'VAT' (Value-Added Tax).
'can' placed inside 'vat' is 'VACANT'.
(Other definitions for vacant that I've seen before include "Unoccupied - unthinking" , "blankly incurious" , "as gents often are!" , "... Blank" , "Empty, not occupied" .)